This means that the NULL indicator will not be accepted into columns belonging to this domain and that the value must be a character string of six digits.

If the default value is defined as being outside the check constraint this ensures that an explicit value must always be inserted into the column:

Searching and Check Clauses

Specification of a CHECK clause means that only values for which the specified search condition evaluates to true may be assigned to a column belonging to the domain.

The search condition, see the Mimer SQL Reference Manual, Search Conditions, in the CHECK clause may only reference the domain values (by using the keyword VALUE), constants, or the keywords CURRENT_USER, SESSION_USER and NULL.